1. Field of the Invention
This invention relates to a projection exposure apparatus for use when for example, the minute pattern of a semiconductive element, a liquid crystal display element or a thin film magnetic head is to be formed.
2. Related Background Art
A projection optical system for use in a projection exposure apparatus is incorporated into the apparatus by way of the super-precise working of a glass material and precise assembling and adjusting steps. Nowadays, in the manufacturing process for semiconductive integrated circuits or the like, use is made of a stepper in which a reticle (a mask) is illuminated with the i-ray (wavelength 365 nm) of a mercury lams as illuminating light and light transmitted through a circuit pattern on the reticle is imaged on a photosensitive substrate (a wafer or the like to which photoresist is applied) through the projection optical system. Also, for evaluation or research, use is made of a so-called excimer stepper which uses an excimer laser (such as a KrF laser of wavelength 48 nm) as illuminating light. In a projection optical system for the excimer stepper, when it is constructed of only a refracting lens, usable glass materials are limited to quartz, fluorite, etc.
Generally, to transform a minute reticle pattern faithfully to a photosensitive substrate by exposure using a projection optical system, the resolving power and depth of focus (DOF) of the projection optical system are important factors. Among projection optical system which have been put into practical use, there is one for i-ray which has a numerical aperture (NA) of the order of 0.6. When the wavelengths of illuminating lights used are the same, if the numerical aperture of the projection optical system is made great, the resolving power thereof will be correspondingly improved. However, the depth of focus (DOF) decreases with an increase in the numerical aperture (NA). When the wavelength of illuminating light is .lambda., the depth of focus is defined as DOF=.+-..lambda./NA.sup.2.
FIG. 1 of the accompanying drawings schematically shows the imaging optical path of a projection optical system according to the prior art, and the projection optical system is comprised of a lens system GA as a front group and a lens system GB as a rear group. As the projection optical system of this kind, one in which both of the reticle R side and the wafer W side are made telecentric or one in which only the wafer W side is made telecentric is popular.
Now, in FIG. 1, suppose any three points A, B and C on the pattern surface of a reticle R (the object surface of the projection optical system). Of rays of light L1, L2, L3, La, La' and La" travelling in various directions from the point A, the ray of light L1 is created at such an angle that it cannot enter the lens system GA as the front group. Also, of the rays of light which have entered the lens system GA as the front group, the rays of light L2 and L3 cannot pass through a pupil ep located on a Fourier transform plane FTP in the projection optical system. On the other hand, the other rays of light La, La' and La" pass through the pupil ep into the lens system GB as the rear group, and are converged at a point A' on the surface of a wafer W (the image plane of the projection optical system). Accordingly, of the rays of light created from the point A on the reticle R, the rays of light passed through the pupil ep a circular area centering around an optical axis AX) of the projection optical system contribute to form a point image at a point A'. Here, of the rays of light travelling from the point A toward the point A', the ray of light La passing through the center point CC (a position on the optical axis) of the pupil ep is called the principal ray, which is parallel to the optical axis AX in the spaces on the object surface side and the image plane side, in the case of a projection optical system in which both sides are telecentric.
This also holds true of the rays of light created from the points B and C on the reticle R, and only the rays of light passed through the pupil ep contribute to the formation of point images B' and C'. Likewise, rays of light Lb and Lc travelling from the points B and C, respectively, in parallel to the optical axis AX and entering the lens system GA both become principal rays passing through the center point CC of the pupil ep. Thus, the pupil ep is in Fourier transform and reverse Fourier transform relations with the pattern surface of the reticle R and the surface of the wafer W, respectively, and of the rays of light from the pattern on the reticle R, all of the rays of light which contribute to imaging pass through the pupil ep while being superposed one upon another.
The numerical aperture of such a projection optical system is generally represented as the wafer side value. In FIG. 1, of the rays of light contributing to the formation of the point image A', the angle .theta..sub.w the rays of light La' and La" passing through the outermost portion of the pupil ep form with the principal ray La on the wafer W corresponds to the numerical aperture NAw on the wafer (image plane) side of this projection optical system, and NAw is expressed as NAw=sin.theta.w. Accordingly, the angle .theta.r the rays of light La' and La" form with the principal ray La on the reticle R side corresponds to the numerical aperture NAr on the reticle (object surface) side, and NAr is expressed as NAr=siner. Further, when the imaging magnification of the projection optical system is 1/M (in the case of 1/5 reduction, M=5), there is the relation that M.multidot.NAr=NAw.
Now, to enhance the resolving power, the numerical aperture NAw (NAr) can be made great, and this is nothing but making the diameter of the pupil ep, i.e., the effective diameters of the lens systems GA and GB, great. Also, the depth of focus DOF decreases in inverse proportion to the square of the numerical aperture NAw and therefore, even if a projection optical system of high numerical aperture could be manufactured, the necessary depth of focus would not be obtained, and this would be a great hindrance in practical use. When for example, the wavelength of the illuminating light is 365 nm and the numerical aperture NAw is 0.6, the depth of focus DOF is about 1 .mu.m (.+-.0.5 .mu.m) in terms of width. Thus, unsatisfactory resolution will be caused in a portion of a shot area (about 20 mm-30 mm square) on the wafer in which the unevenness or curvature of the surface is greater than DOF. Further, there will arise the necessity of effecting the focusing, levelling, etc. for each shot area particularly highly accurately and thus, the burden of a mechanical system, an electrical system and software (the effort to improve measurement resolving power, servo control accuracy, set time, etc.) will increase.
So, the applicant proposed in Japanese Laid-Open Patent Application No. 4-101148, Japanese Laid-Open Patent Application No. 4-22535, etc. A new projection exposure technique which can obtain both of high resolving power and a great depth of focus without using the phase shift reticle disclosed in Japanese Patent Publication No. 62-50811. This exposure technique uses the existing projection optical system to control the illuminating method for a reticle into a special form to thereby increase the resolving power and the depth of focus, and is called SHRINC (Super High Resolution by Illumination Control) method. SHRINC method applies illuminating light to a line and space pattern (L&S pattern) on a reticle at an angle, and passes 0-order diffracted light component and one of .+-.1st-order diffracted light components symmetrically with respect to the center point CC in the pupil ep of the projection optical system, and utilizes the principle of the interference between two beams of light (the interference between one 1st-order diffracted light and 0-order diffracted light) to produce the projected image (interference fringe) of the L&S pattern. According to imaging which utilizes the interference between two beams of light, the occurrence of wave front aberration during defocus is suppressed more than in the prior art (vertical illumination) and therefore, the depth of focus becomes great.
However, SHRINC method can obtain its intended effect when the pattern formed on the reticle, like L&S pattern (grating), has periodic structure, and cannot obtain its effect for an isolated pattern such as a contact hole. Generally, in the case of an isolated minute pattern, diffracted light therefrom is created as Fraunhofer's diffraction almost uniform with respect to the angle of diffraction. Therefore, in the pupil of the projection optical system, the diffracted light is not distinctly separated into 0-order diffracted light and higher-order diffracted light, but is distributed substantially uniformly.
So, as an exposure method for enlarging the apparent depth of focus for an isolated pattern such as a contact hole, a method of dividing exposure for a shot area into a plurality of cycles and moving a wafer by a predetermined amount in the direction of the optical axis during each cycle of exposure was proposed, for example, in U.S. Pat. No. 4,992,825. This exposure method is called FLEX (Focus Latitude Enhancement Exposure) method, and can obtain a sufficient depth of focus enlarging effect for an isolated pattern such as a contact hole. However, FLEX method makes it requisite to multiplexly expose a slightly defocused contact hole image and therefore, the resist image obtained after development necessarily becomes reduced in sharpness. The problem of this reduction in sharpness (the aggravation of profile) can be made up for by the use of resist of a high gamma value, by the use of multilayer resist or by the use of CEL (Contrast Enhancement Layer).
Also, as an attempt to enlarge the depth of focus during the projection of a contact hole pattern without moving a wafer in the direction of the optical axis during the exposing operation as in FLEX method, there is known Super-FLEX method announced in the collection of papers 29a - ZC - 8 and 9 of the meeting of the Applied Physical Society, spring, 1991. Super-FLEX method provides a transparent phase plate (the phase difference being .pi. [rad]) in the pupil of a projection optical system, and provides such a characteristic that the complex amplitude transmittance given to imaging light by this phase plate sequentially varies from the optical axis toward the marginal portion.
In the prior art described above, FLEX method and Super-FLEX method can obtain a sufficient depth of focus increasing effect for an isolated contact hole pattern. However, for a plurality of contact hole patterns which are close to one another to a certain degree, there is a problem that unnecessary film decrease is caused in the photoresist between the contact holes. Also, FLEX method in which a wafer is continuously moved in the direction of the optical axis during the exposing operation is difficult to apply to an exposure apparatus of the scanning exposure type. Further, FLEX method in which exposure to a shot area is divided into first exposure and second exposure and during each exposing operation, a wafer is moved by a predetermined amount in the direction of the optical axis suffers from the inconvenience that the reduction in the processing ability is great and throughput is remarkably reduced.